Radar has a long history of use for military and law enforcement target detection. Due to the very nature of many military and law enforcement missions, the radar user may frequently be exposed to dangerous conditions. There are two types of conventional noise radar systems that are currently available and widely used. One is the ultra wideband band (UWB) noise radar, and the other type is the pulsed noise radar. Each type of radar offers different advantages.
To date, there has been extensive developmental work done with the random signal, UWB, and pulsed waveforms, and each waveform provides certain performance advantages, but they also suffer from a number of drawbacks, disadvantages, and limitations of their waveform characteristics.
The main advantage with the random signal waveform is that this waveform masks its modulated signal characteristics with noise characteristics so that the signal seems to be a channel that is common to all communications systems. Of course, the modulated signal also has a specific structure that can only be deciphered by its intended receiver.
In general, noise radar possess an inherently non-deterministic signal structure that affords a certain level of protection against non-intended, second party receivers. Furthermore, using UWB technology with noise radar offers additional levels of protection since users of such systems can reduce the overall transmit energy during radiation intervals. UWB noise radar requires optical delay lines or matched-filter banks to laboriously assess the presence of a target in each range gate, which is a very time-consuming and cumbersome process that can lead to stale target information.
The UWB waveform also suffers from a few disadvantages and shortcomings. For example, a large number of the UWB noise radar systems implement a linear frequency modulated continuous wave transmit signal. In its most primitive form, a continuous wave signal can be represented by a pure sinusoid, which, when transformed in the frequency domain, results in a discrete frequency response centered about its carrier frequency. However, centering the response on the carrier frequency sacrifices a high resolution capability, which oftentimes results in the inability to resolve reflected signal components subject to channel fading, and so on. Other difficulties with the continuous wave signal are the complex hardware requirements associated with transmit/receive isolation and separate transmit-receive entities, the need for phase detectors to extract the target range, and the inability to use a multi-channel array. Similarly, the pulsed waveform also suffers from the fact that they impose a high transmit power requirement on their amplifiers. This suggests that more expensive, stable amplifiers are mandatory in order for pulsed waveform implementations to work properly.
Moreover, the typical military and law enforcement mission frequently includes critical security and surveillance requirements that can make unwanted detection both undesirable and dangerous, and the prior art radar arrangements oftentimes fail to satisfy those critical requirements. Thus there has been a long-felt need for a radar signal generation apparatus that overcomes the extensive, costly, and time-consuming hardware disadvantages of prior art systems such as optical delay lines and matched-filter banks, as well as the waveform shortcomings of centering the carrier frequency response and the risks of unwanted detection during extended range surveillance operations.
The concept of noise radar was first introduced in the 1950s by B. M. Horton where challenges surrounding his proposal were predominately due to hardware requirements needed for such a system to work affectively [1]. Due to modem hardware advances, the novelty of noise radar has begun to generate some recent interest. In fact, there has been a reemergence in research focused on using noise as an RF source. Narayanan et al., for example, designed and tested an ultra wideband (UWB) random noise radar (RNR) operating in the 1-2 GHz band at 200 meters in range. Results from this work established a solid basis for the feasibility of using noise as an RF source for the purposes of detecting targets [2]-[6]. Extensive work conducted by the Nanjing University (China) resulted in conclusive analysis describing an ideal waveform hybrid made up of UWB and random signal radar (RSR) waveforms where the authors determined that the best way to achieve a low probability of intercept/low probability of detect (LPIILPD) system with low power and high resolution would be to combine these two waveforms [7]-[9]. The Russian Academy of Sciences has done extensive exploration on the plausibility of using code spectral modulation for communications systems, a technique first introduced in 1965 by J. L. Poirier [10], in order to insert data onto a random signal carrier where the concept of double spectral processing is exercised to extract the time delay corresponding to the information symbol [11]. Despite the availability of noise radar technology, the broader radar community has not yet fully embraced its attributes. The productivity of this work hopes to shorten this gap. This research is motivated by the belief that a strong majority of conventional radar systems transmit energy openly. This implies an obvious susceptibility to some advanced, non-intended 2nd party passive receivers who might have an interest in understanding what the unsecured RF source's intentions are. It was therefore felt that some research in this area could serve as a means to suggest one possible solution for addressing this concern. It was also felt that the productivity of the research should appreciate two fundamental criteria: a) the proposed waveform should not impede the radar's primary functions e.g. target detection, terrain mapping, and b) the scope of incorporating the proposed waveform should be done with marginal impact to the host waveform structure.
Due to the nature of noise, both its time and frequency domain responses are random making it virtually impossible for the transmitted signal to be recognized and/or corrupted by non-intended 2nd party passive receivers without having a priori knowledge of the exact structure of the noise signal. However, transmitting noise alone is insignificant unless a carrier signal of specific frequency and phase is used as a reference. One practical approach is to do as Narayanan et al. did and artificially inject time-delayed replicas of the signal into the processing side of the system architecture. This was done in an effort to institute some form of coherence into their waveform while also reducing the overall computational complexity involved with signal processing. By selectively choosing specific time-delays, they in essence only needed to consider the presence of targets at certain ranges. As a result, when the radar returns were present in the candidate range bins, a well correlated signal would result. While what Narayanan et al. performed proved to be quite useful for the noise radar community, their approach only considered a short range test <<10 m) and a longer range test (200 m). Furthermore, their experimental set up suggests that specific hardware needs to be incorporated in order to successfully implement a UWB random noise radar. Because a majority of radar in use today implements a pulse-Doppler waveform structure, Narayanan's approach would impose a drastic impact to the architecture of these types of systems. These points clearly violate the fundamental criteria set forth earlier.
For these reasons, we consider herein a different noise radar approach that employs a composite waveform generated by amplitude modulating a complex pulse-Doppler waveform using noise. This waveform will serve as the basis for the inception of a newly defined noise radar class referred to by the authors as Noise Correlation Radar (NCR). It distinguishes itself from its peers, namely RNR and RSR, since neither a pure noise signal nor a modulated noise signal is transmitted. Instead, a pulse-Doppler waveform using pulse compression and linear frequency modulation (LFM) techniques serves as the host or modulated waveform. The LFM provides the coherence needed for proper correlation during the signal processing as well as the bandwidth necessary for resolving targets. It will be shown that the noise modulated pulse-Doppler waveform embodies the ideal characteristics of both noise radar and pulse-Doppler radar waveform in that it will have a chirped bandwidth (BW), pulse compression gain, masked transmit spectrum, and ideal ambiguity function. Notably, these facts appeal to many radar applications.